1. Field of the Invention
The present invention relates generally to direct injection spark ignition (DISI) internal combustion engines. More particularly, the invention relates to a method and system for operating a DISI internal combustion engine having a plurality of combustion and compression ratio operating modes.
2. Background Art
Direct injection spark ignition engines can be operated in a homogeneous combustion mode during high load operating conditions, and a stratified combustion mode for lean burn during low or part load operating conditions. In the homogeneous combustion mode, fuel is typically injected into the combustion chamber early in the intake stroke to allow sufficient mixing with an air charge prior to ignition. In the stratified mode, fuel is injected later in the compression stroke such that complete mixing is not achieved prior to ignition. Transitions between combustion modes often result in air/fuel (A/F) ratio discontinuities at the mode switches.
In the absence of adequate controls, such A/F discontinuities often result in objectionable transition torque disturbances that may be perceived by an operator. These torque disturbances can be mitigated as described for example in U.S. Pat. No. 5,947,079 by coordinating the engine electronic throttle, fuel flow and ignition timing. When transitioning from stratified to homogeneous, for example, manifold pressure may be decreased (throttle closed) and a constant torque maintained (fuel flow and spark retarded) until a manifold pressure is attained that corresponds to an achievable homogeneous operating point. At this point, the stratified-to-homogeneous transition is initiated by switching from late (compression stroke) to early (intake stroke) injection timing, and the fuel flow and spark advance are simultaneously adjusted to attain constant torque in the homogeneous mode. A similar approach is used to manage the homogeneous-to-stratified mode transitions.
During a stratified-to-homogeneous mode transition, however, it may be noted that an acceptable homogeneous mode torque may be achieved at an air/fuel (A/F) ratio greater (leaner) than the stratified operating point at which the mode transition is initiated. Consequently, the engine remains susceptible to misfire upon transition. Further, the combination of homogeneous lean-limit A/F and ignition timing constraints may not permit disturbance-free transitions at all operating points.
In view of the prior art, the inventors herein have recognized that the ability to control an engine""s compression ratio provides an additional means by which to manage constant-torque mode transitions of a DISI internal combustion engine.
A method of operating a direct injection spark ignition (DISI) engine is disclosed having the steps of determining a demanded torque output of the engine, determining a current combustion mode and a current compression ratio mode of the engine, transitioning operation of the engine from the current combustion mode to a new combustion mode to produce the demanded torque output, and transitioning operation of the engine from the current compression ratio mode to a new compression ratio mode so as to minimize torque disturbances during the transitioning of engine operation from the current combustion mode to the new combustion mode. The DISI engine is provided with a variable compression ratio apparatus, or equivalent means, to vary the engine""s compression ratio during combustion mode transitions.
An advantage of the above-described method, for example, is that torque disturbances resulting from transitions between homogeneous and stratified modes of operation can be minimized by transitioning from one compression ratio mode to another. As such, control of a variable compression ratio apparatus can be used to improve transition control from homogeneous to stratified modes of operation and visa-versa. Exemplary, non-limiting methods are disclosed herein for transitioning from stratified/high compression to homogeneous (high or low compression), homogeneous (low or high compression) to stratified/high compression, homogeneous/high compression to homogeneous/low compression, and homogeneous/low compression to homogeneous/high compression.
In a related aspect of the present invention, a corresponding system is also provided having a variable compression ratio apparatus. In accordance with a preferred embodiment, the system further includes a controller and corresponding computer and computer code for determining a demanded torque output of the engine, determining a desired combustion mode and desired compression ratio mode of the engine, transitioning engine operation to the desired combustion mode in order to produce the demanded torque, and transitioning engine operation as required from a first compression ratio mode to a second compression ratio mode so as to minimize torque disturbances during the transitioning of engine operation to the desired combustion mode.
Further advantages, objects and features of the invention will become apparent from the following detailed description of the invention taken in conjunction with the accompanying figures showing illustrative embodiments of the invention.